The present invention relates to an extrusion die plate and a method of making same. More specifically, the invention is directed to a die plate of greater wear resistance and suitable for use in the extrusion of plastics containing fillers, particularly fillers which are relatively hard and have a tendency to be abrasive.
In the extrusion of synthetic resins and similar plastic materials to form pellets, the extruded material is severed into the appropriate size pellets by a knife or blade which periodically wipes across the discharge side of the extrusion die plate, the wiping action resulting in a severing of the extruded molten plastic into generally cylindrical pellets as the resin exits the die plate. The wiping or cutting action of the cutting blades or knives across the external or discharge face of the die plate, causing the discharge face to wear rapidly. To overcome this excessive wearing, it is common practice in constructing die plates to provide a facing on the discharge face which is extremely hard and abrasion resistant. Thus, facings of materials such as tungsten carbide, ceramics or other extremely hard, abrasion resistant materials are commonly employed. While the use of such abrasion resistant materials substantially reduces the wearing problems caused by the rotating cutting assembly, other problems are presented by the use of such materials. The hard, abrasion resistant materials are intrinsically brittle, not easily machinable and susceptible to slivering, chipping, etc. Accordingly, attempts to drill holes in such materials to form the extrusion ports or passageways usually result in slivering, splintering or chipping. When this occurs, the extrusion port becomes irregular in cross-sectional shape and consequently the pellets are likewise not uniform in cross-sectional shape or size. However, such uniformity is necessary so that the pellets may be accurately measured by volume.
Die plate construction aimed at circumventing the above described problem is described in U.S. Pat. No. 3,599,285 to Hamilton. The Hamilton patent discloses a die plate in which the relatively soft, machinable base metal which forms the main body of the die plate is used to form a series of protruding nozzles which terminate at the discharge face of the die plate. The space between the nozzles is filled with a facing of tungsten carbide or some other similar hard and abrasion resistant material. Thus, smooth, uniform extrusion ports can be drilled or otherwise formed in the machinable base metal forming the main body of the die plate. On the other hand, since the major portion of the discharge face is comprised of hard tungsten carbide or similar abrasion resistant material, the die plate discharge face will not wear excessively. U.S. Pat. No. 3,271,822 to Rhino discloses a die plate in which the extrusion orifices and a portion of the extrusion ports are formed by a plurality of discreet wafers of tungsten carbide which are brazed to the die plate body.
While the die plate disclosed in the Hamilton patent is a vast improvement over prior art die plates, it suffers from the infirmity that because the extrusion orifices are formed by the relatively soft base plate material, the orifices and extrusion ports immediately adjacent thereto tend to wear rather rapidly which again results in the production of non-uniform sized pellets. Such wearing is particularly acute when resins loaded with inorganic or other hard fillers are being extruded. The Rhino et al die plate requires the use of tungsten carbide wafers having preformed extrusion orifices or apertures which must be individually positioned on the die plate face such as by brazing or the like. Moreover, because the extrusion ports and orifices of the Rhino die plate are formed by tungsten carbide, a material which is extremely brittle and which can not be machined easily, the likelihood of chipping or slivering of the extrusion orifice or port is increased.